Automatic gain control circuit



June 30, 1936 K. WILHELM ET AL. 2,046,141

AUTOMATIC GAIN CONTROL CIRCUIT Filed April 21, 1934 wvw INVENTORS KARL Wl LH ELM BY ERN T LOTZ ATPORNEY Patented June 30, 1936 UNITED STATES PATENT OFFIQE 2,046,141 AUTOMATIC GAIN CONTROL CIRCUIT Application April 21, 1934, Serial No. 721,722 In Germany May 30, 1933 4 Claims.

The object of the invention is a radio receiver set comprising means designed to regulate fading action. The purpose of the present invention is to so design the receiver that its sensitiveness, or responsiveness, will be low for'very small carrier waves, and that a certain change in the gain or amplification will be efiected only after a definite threshold value has been exceeded, and which, as the carrier wave grows, will occasion a further reduction of the sensitivenss of the receiver. In this way conditions are made so that the receiver will handle and respond to such waves only that go considerably beyond the general stray level, with the result that the repro duction of radio programs will be both of high quality and sufficiently free from disturbances.

Radio receivers of the kind known in the prior art comprising means adapted to insure automatic anti-fading regulations involve the drawback that also incoming waves of any desired feebleness at all would be reproduced, including those which, in undisturbed reception, would fail to furnish sufiicient output volume of sound. In this way, also, the disturbing actions being or" an order of magnitude close or equal to weak incoming sound volumes are reproduced, with the result that in the reproduction of very feeble transmitters predominantly the disturbing or background noises traceable to the general stray level were found to arise. Now, a receiver apparatus of the kind disclosed by this invention will not reproduce programs of transmitters that do not rise above the level conditioned by the output sound volume, and thus also fails to reproduce strays of like field strength, but it will respond to transmitters of appropriate strength and constant sound volume. In other Words there is thus secured a sort of selective action regarding the transmitter stations to be handled and reproduced.

In a receiver set comprisng anti-fading means there is used, according to this invention, for the rectifier whose output currents are utilized for regulation a negatively biased diode. In the output circuit of the path of the diode tube, therefore, only currents will be able to flow at all whenever the amplitudes supplied to the diode path are able to overcome the negative biasing voltage of the diode path. It is only when the incoming amplitudes exceed this value that a current will be able to flow in the diode circuit, which, in turn, insures the regulation of sensitiveness of the tube stages located below it. The threshold value of response of the diode path conditioned by the negative biasing potential of the diode path will be made particularly marked if the radio amplifier tubes regulating the signal strength are operated on a positive biasing potential. In an amplifier tube worked on a positive biasing potential a grid current arises in the input circuit which occasions damping in the input circuit, and thus lack of sensitiveness of the amplifier action of the tube stage. By the influence of the direct current flowing in the diode path, a drop of potential is created across the resistance contained in the input circuit which is able to overcome the positive biasing voltage.

An exemplified embodiment of the invention is shown in the drawing, and in it the numeral I denotes the radio amplifier tube to be regulated, and to whose control grid the input circuit comprising the coil L and the rotary condenser C are connected. With the grounded cathode the input circuit is united by way of a blocking condenser K. Contained in the plate circuit are the usual coupling circuits which are conventionally shown in the drawing. Numeral 2 denotes a diode rectifier whose external circuit contains an oscillation circuit consisting of L1 and C and a resistance R bypassed by a blocking condenser K1. From the end of the said resistance turned towards the anode, a lead is brought by way of the resistance R to the cathode end of the input circuit of the tube 1.

The rectified or demodulated voltage is derived across points 5, 4. To the cathode of the diode tube is imparted a positive biasing voltage to ground, say, from a battery B which is shunted by a condenser K2. The operation of the circuit scheme here disclosed is as follows:

If no signal voltages are impressed upon the circuit LC, the grid of tube I separated from the cathode will experience a biasing potential of around zero value; in other words, there flows such a large grid current that the fall of potential occasioned thereby across the resistances R and R will be just about equal to the positive biasing potential of the battery B. If resistances R and R be made alike, there will prevail a biasing voltage across the points 6 and 4 of a value equal to one-half the battery Voltage; to be more precise, point 6 presents a negative biasing voltage in reference to point 4. However, this means nothing else but that the signal voltages fed to the diode must first surmount, or exceed, the value of the negative biasing potential before the diode will be capable at all of entering into operation.

As a result of the flow of the grid current the oscillation circuit LC is subject to marked damping, and asa result the amplification is diminished. If, then, a signal of an amplitude reaches the input circuit whose value, when amplified," is large enough to surmount the negative biasing voltage of the diode, a direct current will fiow in the external circuit of the diode, with the result that the negative biasing potential prevailing at point 6' compared with point 4 is increased. As

a result, also, the grid of the tube 1 receives a negative bias, and thegrid current diminishes.

the cathode of diode 2 in reference to ground may be brought about also by the adoption of other ways and means, for instance, in the form of a fall of voltage'across a resistance. The current V which is causative of this fall of voltage may be derived from one or moretubes. Moreover, the diode may be confined within one and the same glass envelope with the audio amplifier next 7 above.

The rectified alternating current component is fed by way of a condenser K3 to a potentiometer 3, 4, whose contact slide 5 is associated with the control grid of the followingtube. There is afurther' chance that will be found appropriate in practice; i. e., to produce additional level regulation by the agencyoi a gain or volume control device manipulable by hand. Hence, the present invention is adapted to insure the aim and problem ofthe invention to make the responsiveness or sensitivity of the receiver apparatus low for very low carrier waves, and to occasion'a certain .change in the gain or amplification only after a predetermined threshold value has been exceeded. Of course,,between the amplifier I and detector 2 there may be interposed additional stages of amplifiers. p t Y 7 While we have indicated and described a sys-' tem for carrying ourinvention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may bemade without departing from the scope of our invention, as set forth in the appended claims.

What is claimed is:- r l 1. In combination with a signal amplifier having a signal input circuit connected between its signal control grid and cathode, a diode rectifier coupled to the output circuit of the amplifier, a direct current path connected between the grid and cathode of the amplifier and including an impedance in series with a source of direct current, the diode electrodes being connected to different points on said impedance, the amplifier 'grid being connected to the diode anode point on the impedance and the amplifier cathode being connected to the negative side of said source, and said sourcehaving avoltage magnitude such that thefamplifier grid draws current in the absence of signals above a predeterminedamplitude and imparts a minimum sensitivity to the amplifier, and said impedance having a resistive value to provide a diode anode bias voltage from said grid current sufiicient to render the rectifier in0perative for signals below said amplitude.-

2. Ina system as defined in claim 1, means coupled to said impedance for deriving therefrom the audio'component of rectified signal voltage' developed across the impedance.

3. In a system as defined in claim 1, a second current path, and anautomatic gaincontrol connection between the amplifier input grid and a point on said impedance which assumes a negative potential when signals are rectified by the rectifier, and said improvement comprising a source of invariable direct current voltage connected between a point on the impedance, which is positive with respect to the first point, and the amplifier cathode, said voltage source being arranged to establish a potential relation between the amplifier grid and cathode such asto cause the amplifier grid to draw grid current in the absence of signals above a desired amplitude, and

said impedance having a magnitude such that' said grid current develops a bias voltage thereacross of a value sufficient to prevent the fiow of the rectifier space current. V f

' a ERNST KLOTZ.

KARL WILHELM. V 

